The present invention relates to corrosion resistant coatings for metal surfaces cured by the means of actinic radiation. These coatings are intended to protect metal surfaces in a variety of corrosive environments, and particularly, in a salt water environment.
Corrosion is an electrochemical process which leads to the deterioration and eventual destruction of exposed metal surfaces. The presence of conducting electrolyte, moisture and oxygen to successfully complete the electric circuit on the thermodynamically unstable metal surface are the main factors of a corrosion process. The mechanism of steel corrosion can be illustrated by a series of electrochemical reactions which involve iron (Fe), according to the mechanism outlined in "Corrosion Basics: An Introduction" published by NACE International, Houston 1984. Metal dissolution takes place at the anode in the course of the oxidation reaction: EQU Fe.fwdarw.Fe.sup.2+ +2e.sup.- [1]
Released electrons can migrate to a cathodic site either through steel or via electrolyte. There they react with available water and oxygen. In neutral or basic conditions hydroxyl ions are produced: EQU 2H.sub.2 O+O.sub.2 +4e.sup.-.fwdarw.4OH.sup.- [2]
Hydroxyl ions then recombine with ferrous ions producing corrosion products (red rust upon further oxidation): EQU Fe.sup.++ +2OH.sup.-.fwdarw.Fe(OH).sub.2 [3]
One of the most efficient ways to thwart corrosion is to shield metal surfaces from the environment with protective coatings. These coatings are of great importance for numerous civilian and military uses. The range of applications is extremely broad: ship hulls and topside exterior surfaces; bridges and supports; various fuel, potable water, chemical and sewage tanks; numerous structural and building uses, etc.
The type and level of coating protection needed is determined by many factors including the environmental exposure conditions (salt water, UV, temperature, chemicals, oil and greases), use and handling of the part (toughness, scratch resistance, impact), service life, etc.
Over the years coatings based on alkyd, urethane, epoxy and other technologies have been developed. However, many of these coatings use organic solvents (VOC's, Volatile Organic Compounds) which present environmental, energy and safety concerns. For example, epoxy/amido-amine modified polyamide cured paints are discussed by Hare, "Protective Coatings" Technology Publishing Co. Pittsburgh, Pa. 1994. These paints reportedly give 6 years of service in marine immersion with slightly reduced service life in ballast tank areas. Water and corrosion resistance of these paints is attributed to a high degree of crosslinking and excellent adhesion to steel which prevents or significantly delays the corrosion process from starting. In addition, efficient corrosion protection can be attributed to good substrate wetting by the amide linkages and high peel strength. Epoxy/polyamide systems are easily recoatable and can be repeatedly cleaned without deterioration. However, these paints are solvents based, take long time to cure, require accurate two-part mixing, have a limited pot life, and their cure is temperature dependent. Consequently, new approaches in the coating industry to reduce VOC's are often driven by regulatory, environmental, productivity and related issues.
Recent and current efforts center around water based, powder, high solids, radiation cure and other coating technologies. Some of them have resulted in environmentally friendly coatings that meet many of the desired requirements.